Dust produced during the handling, transport, storage, crushing, and utilization of coal and petroleum coke is not only a nuisance, it is a major health and safety issue facing the petroleum and coal industry. Fires, explosions, and black lung cost the petroleum, mining, utility and steel industries millions of dollars annually due to lost production, medical expenses, lost equipment, and high insurance premiums. The control of coal and coke dust is imperative, as can be seen from increased internal control efforts at plants that handle these materials and from increased government enforcement of air quality particulate standards.
Dust from coal or coke is produced and airborne by two major mechanisms--impact or wind erosion. A ton of coke or coal may travel at 20 miles per hour on a refinery conveyor belt only to be dumped onto a transfer chute. The impact can produce a substantial amount of airborne particulate matter. Fine particulates also are produced during stacking, crushing, milling, breaking, or reclaiming operations. Whenever coal or coke is in motion and subjected to impact, dust is generated.
One method that has been used in the past to control such dust is by spraying the coal or coke with water. Water is effective to prevent dusting; however, the water evaporates rather quickly. Therefore, the coal or coke must be sprayed almost continuously in order for water to be an effective anti-dusting agent. This continuous spraying is cumbersome and costly, and the droplets of water in the spray often do not even capture the finest, most hazardous particulates. Furthermore, not all coals can be wetted using water, and, in some uses, moisture level restrictions will be exceeded if too much water is used. Also, an economic penalty may result because it is less efficient to burn high moisture coke or coal. In addition, water has little, if any residual effect. Once applied, water generally does not continue to reduce dusting at downstream operations and evaporates quickly from pile or storage surfaces. Surfactants can be added to the water to improve coal wettability in some instances; however, the problem of evaporation remains. Because petroleum coke is even more difficult to wet than coal, water is an even less effective anti-dusting agent when used with coke.
Organic chemicals, such as lauryl sulfate, alkanol amide, and oil, which may be blended with wax, asphalt, or other bitumens, also have been used to prevent dusting. These organic chemicals are effective to avoid the need to continuously spray the coal or coke because they control fugitive dust at very low concentrations. However, like water, organic chemicals tend to have little residual effect. Therefore, organic chemicals generally do not continue to reduce dusting at downstream operations. Furthermore, environmental regulations governing the sale and use of waste oils have tended to decrease the use of oil as an anti-dusting agent.
Another dust control method is the use of foam comprised of air, water, and a foaming agent. The addition of a foaming agent to the water generates a large volume of foam from a small amount of water. Therefore, the addition of a foaming agent allows for dust control using much less water. For example 0.5 gallons of water may be expanded to form 15 to 20 gallons of foam. And much less foam is required to achieve the same anti-dusting effect as water. Like water, a high pressure steam of foam "knocks-down" a fair amount of the larger particulates in the coal dust. When the foam bubbles impact the dust particles, the particles are wet by the imploding bubbles and captured. Many fine droplets also are released to scrub more fine particles. Although foam is effective to control fugitive dust, and even has a short term residual effect, once the water in the foam has evaporated, the residual effect essentially disappears.
Encrusting agents or chemical binders, such as latex or lignosulfonates which result from the processing of wood for paper products, also have been used to coat piles of coal to reduce wind-eroded fugitive emissions. However, encrusting agents and chemical binders usually are used as surface coatings only, which do not reduce the emissions that may result during later handling of the stored coal.
A significant amount of effort has been devoted to finding an additive which would treat all of the coal or coke during active storage to prevent dusting on very active piles. Highly dilute latexes, oil, emulsions, flocculants, molasses, and lignosulfonates have been used for this purpose; however, weathering, biodegradation, and adsorption into the coal have frustrated many of these attempts. Thus, it is apparent that a product is needed which would reduce in-transit losses and dusting, and which would continue to suppress dusting throughout the off-loading and handling operations at the plant.